1. Field of the invention
This invention relates to electrolytic manganese dioxide to be suitably used for alkaline, manganic and other batteries and a method of manufacturing the same. More particularly, the present invention relates to electrolytic manganese dioxide that can effectively improve the initial performance and the storability of batteries comprising the same and a method of effectively and efficiently manufacturing such electrolytic manganese dioxide on an industrial basis.
2. Prior art
The overall performance of an alkaline or manganic battery heavily relies on its initial performance and storability. A battery of the type under consideration normally comprises an anodic composite, cathodic zinc, electrolyte, a separator and a collector. The anodic composite by turn normally comprises as principal components manganese dioxide, electroconductive materials and electrolyte. Electrolytically, naturally or chemically produced manganese dioxide can be used for the battery.
While the performance of a battery is a function of the performances of its individual components including manganese dioxide, the contents of the different components and other factors, it is of vital importance for a battery to contain manganese dioxide as much as possible as a component of the anodic composite within a limited space if an improved performance is required for the battery.
To meet the requirement of increasing the manganese dioxide content per unit volume of a battery, particularly in the case of electrolytic manganese dioxide to be used for the anodic composite, efforts are being made to optimize the average particle size and the distribution of differently sized particles of electrolytic manganese dioxide at a post-processing step in the process of manufacturing manganese dioxide by electrolysis. Commercially available batteries typically have a BET specific surface area (hereinafter referred to simply as BET, which is the surface area measured by the Brunauer, Emmett and Teller method) of about 30 to 50 m.sup.2 /g.
On the other hand, in order to improve the storability of a battery, it is essential to use electrolytic manganese dioxide that can effectively suppress generation of gas from the battery while it is being stored, does not significantly contain heavy metal impurities such as iron, lead and copper and is electrochemically active. From the viewpoint of storability of a battery, the average particle size of electrolytic manganese dioxide is also important. If the average particle size is too small, particles of manganese dioxide can become chemically unstable and deteriorated by the electrolyte in the battery while the latter is being stored. Although it is believed that electrolytic manganese dioxide is resistant against deterioration by the electrolyte particularly when the battery is an alkaline battery that is rechargeable for a limited number of times (or structurally a primary battery but can be recharged under certain conditions) and the electrolyte is an alkaline substance, much has to be done to technologically improve the quality of electrolytic manganese dioxide if it is used for batteries.
Known methods of manufacturing electrolytic manganese dioxide (hereinaftre referred to as EMD) include the suspension method (as disclosed in Japanese Patent Publications Nos. 57-42711 and 63-26389), the charge fluctuation method (as disclosed in Japanese Patent Publication No. 5-275054) and the clarification method. The operation of electrolysis is conducted with a current density of 1.0 to 4.0 A/dm.sup.2 and an electrolytic temperature of 94 to 96.degree. C. for the suspension method and with a current density of 0.4 to 0.9 A/dm.sup.2 and an electrolytic temperature of 90 to 96.degree. C. for the clarification method, the H.sub.2 SO.sub.4 and MnSO.sub.4 concentrations typically being 0.35 to 0.6 mol/liter and 0.5 to 1.2 mol/liter respectively for the both methods.